This invention relates generally to the field of rotary slitters used for forming closures for containers particularly those which include tamper-evident rings which are separated from the skirt portion of the closure when the closure is first removed.
The slitting of roll-on closures is an old well-developed art. The slitting of aluminum soft drink caps, for example, is accomplished on a rotary threader using a curved knife blade segment. The severable bridges are the result of slots in the knife blade segment. More slots form more bridges. Wider slots form stronger bridges.
In the prior art rotary slitter, a main dial rotates about a stationary hub. Internal tools are mounted on the dial and move with it. The number of tools is determined by the required number of caps per minute output. Typically, eighty caps per minute per spindle is normal.
A star gear is stationary and mounted on the hub. As the dial rotates, the star gear rotates the spindle eliminating slipping of the internal tool. The closures are picked up by the internal tool, rotated over the knife segment to produce the slots and subsequently discharge. A curved knife segment is mounted on a stationary seat ring on the stationary hub.
There are two major shortcomings when using the above described structure.
A first is the fact that the machines are dedicated to a particular size cap. To make caps of another size requires another machine. Cost is normally not a critical factor where the volume is very high, but can be a major concern in the case of forming food container closures where sizes are larger and volumes are smaller.
A second problem relates to the adjustment of the curved knife which is periodically required. Maintaining even contact with the cap is of paramount importance. As the caps rotate over the knife segment, the cuts and the interconnecting bridges will change as the knife segment is adjusted upwardly or downwardly. To provide a deeper penetration, the segment is shimmed up. This is usually necessary after a sharpening operation. However, when reset, the center of the knife segment is higher than the end portions. When lowered, the end portions are higher. This factor results in uneven bridge strength.